A breakdown voltage of a semiconductor device that is represented by a diode, a MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), and an IGBT (Insulated Gate Bipolar Transistor) includes a reverse breakdown voltage of a diode and an off-breakdown voltage of a transistor, which are both breakdown voltages in a state where the semiconductor element does not function as an active element. In the state where the semiconductor element does not function as the active element, the voltage applied to the semiconductor device is held by a depletion layer that spreads in a semiconductor substrate provided with the element.
As a technique for improving the breakdown voltage performance of the semiconductor device, there is known a technique of arranging a terminating end region including an impurity injection layer of a conductivity type opposite to the semiconductor substrate so as to surround an active area serving as an active element in the semiconductor substrate.
As a structure of the terminating end region (termination structure), there is known a structure of forming, on the outer side of the active area, the impurity injection layer of a conductivity type opposite to the semiconductor substrate in layers so as to be spaced apart from each other toward the outer side. Since the impurity injection layer has an annular shape when the semiconductor device is viewed from above, it is called a guard ring or an FLR (Field Limiting Ring). In the present invention, the structure refers to the entire plurality of guard rings, and is called a guard ring structure.
When such a guard ring structure is arranged, the depletion layer easily spreads toward the outer side of the active area. As a result, the electric field concentration at the bottom end portion of the active area (corner portion of the injection layer when viewed in cross-section) is alleviated, and the breakdown voltage of the semiconductor device can be improved.
In transistors such as MOSFET, IGBT, and the like, the outermost portion of the active area normally becomes a deep impurity injection layer (well) of a conductivity type opposite to the semiconductor substrate. Thus, the guard ring is often formed simultaneously with the well. This is similar in the PIN (P-Intrinsic-N) diode, and the guard ring is normally formed simultaneously with the impurity injection layer (base) that becomes the active area.
As described above, the role of the guard ring structure is to spread the depletion layer toward the outer side of the active area. However, along with this, the electric field concentration occurs not only at the bottom end portion of the active area but also at the outer side bottom end portion of the individual guard ring. Such an electric field concentration tends to become stronger as the change in impurity concentration in the vicinity of the PN junction surface becomes steeper.
Since the well and the base have a relatively high concentration, the change in concentration in the vicinity of the PN junction surface becomes steeper, and strong electric field concentration tends to easily occur. Thus, in Si (silicon), the impurity diffusion is normally promoted by anneal processing performed at high temperature for a long time to make the change in concentration in the vicinity of the PN junction surface gradual, thus alleviating the electric field concentration.
Alternatively, an embedded injection layer having a relatively low concentration may be formed at the bottom end portion of the active area and the outer side bottom end portion of the individual guard ring to change the concentration in a step-wise manner, thus alleviating the electric field concentration. Such prior art is described in Patent Document 1.
In addition, as techniques related to the termination structure, there are techniques disclosed in Patent Document 2 and Patent Document 3.